Recently there has been localized fogging in two corners of my 4 x 5 negatives. This occurs near the loading end of the holder and is greater on the top side of the camera. The fog is like a ray of light coming from the corner and does not always occur. It seems independent of holder The holders are made by Fidelity. The camera is a top rf speed graphic in excellent condition. The graphloc spring is strong. There is some longitudinal movement, 1/8th inch, of the holder in the back. Please help!

You have a light leak... which you may have already extrapolated for yourself. From what you posted, I'd say it will be somewhere in the bellows, probably upper right, though it could also be caused by a leak at the point where the film holder contacts the camera body.

Take the back off the camera and go into a dark room with a penlight. Flash the penlight around inside the bellows and see if you can see a pinhole from the outside. If this proves fruitless, take the lensboard out, replace the camera back, put a film holder in and put the flashlight inside the opening at the front of the camera. Look for light leaks at the film holder.

There is also the possibility that certain film holders are leaking light. I know of no simple test for that, other than to number them all and keep tabs on which negative came out of which holder. Keep records and if you find that certain holders always seem to be the ones with the fogging... you have located your culpret.

Having purchased a $30.00 Speed Graphic myself recently, I suspect I'll be taking my own advice soon. Good Luck.